| Desert steppe is a unique grassland type with the most xeric characteristics in central Asia.It is a very fragile grassland ecosystem in the transition from the steppe to the desert.It has always been a hot area concerned and studied by ecologists,geographers,environmentalists,and economists.Desert steppe in Ningxia accounts for 42%of the grassland area in the whole region.The precipitation is the primary water source,but its annual amount is small,and its spatial-temporal distribution is uneven.The sierozem soil is scattered in the vast aeolian sandy soil,and exposed weathering bedrocks are nested.Artemisia scoparia is the main pioneer and dominant species because of its strong adaptability to harsh environment and strong competitiveness in communities.The growth of companion species are substantially inhibited,resulting in the low utilization rate of the desert steppe.The adaptation strategy reflects the long-term response of plants to different habitats and is an essential prerequisite for elucidating the mechanism of maintaining biodiversity.The adaptation strategy of A.scoparia is a key scientific issue to study the formation mechanism of single dominant community and vegetation restoration mechanism in desert steppe.In this study,the desert steppe in central Ningxia was selected as the research area,and A.scoparia was the research object.Combining plot monitoring,field investigation,simulation test,and data analysis,the ecological adaptation strategies of A.scoparia were discussed from two main aspects:interspecific relationships(niche characteristics and allelopathic effects)and intraspecific characteristics(population dynamics and reproductive characteristics).The main results are as follows:1.Status of A.scoparia in plant communities.Forty-two plant species belonging to 36 genera and 15 families were recorded in the investigation plots,and Leguminosae,Gramineae,Compositae,and Chenopodiaceae were the dominant families.The niche breadth and the entire breadth of A.scoparia were the highest and distributed in the center of the DCA map of the niche overlap index,which indicated that A.scoparia is the dominant species of the area,and occupied a prominent position in the community of desert steppe.2.Allelopathy of A.scoparia.Using aboveground and underground portion of A.scoparia as donor materials,the effects of different concentrations of aqueous extracts on ten kinds of common plants were analyzed.It was found that the sensitivity of seedling growth was stronger than that of seed germination,and radicle length was the most sensitive index to measure the allelopathy of A.scoparia.Based on the sensitivity index of allelopathy,Stipa capillata was most inhibited,while Agropyron cristatum×Agropyron desertorum cv.'Hycrest-Mengnong'(HM)was promoted.Different concentrations of aqueous extracts promoted the growth of Melilotus officinalis,Agropyron cristatum,and HM,but inhibited the growth of Agropyron mongolicum,Lespedeza bicolor and S.capillata.The allelopathic intensity of aqueous extracts from the underground portion of A,scoparia was higher than that of the aboveground portion.By GC-MS analysis,57 chemical components were detected and identified in the aqueous extracts from the aboveground and underground portions of A.scoparia.There were 24 components in both parts,and the largest number were alcohols.More components were identified in the aqueous extracts from the aboveground portion than from the underground portion.Therefore,allelopathy played an essential role in enhancing competitiveness,which is one of the ecological adaptation strategies of A.scoparia in the desert steppe.3.The population dynamic of A.scoparia.Through the compilation of population dynamic life table of A.scoparia in 2017 and 2018,it was found that the survival curve of A.scoparia population tended to be Deevey-I.The survival of individuals of A.scoparia under different soil types and precipitation treatments showed a downward trend in the early stage,and the fluctuation of mortality was more considerable in the aeolian sand soil and calcareous soil.The interaction between soil types and precipitation treatments only had a significant impact on plant height and crown width.The plasticity indices of plant height and crown breadth were higher,which indicated that A.scoparia could respond to habitat pressure by giving priority to the two parameters.4.The reproduction strategy of A.scoparia.The aboveground and underground biomass,vegetative and reproductive organ biomass,number and the weight of reproductive body of A.scoparia on the weathered residual soil were less than those of aeolian sand soil and calcareous soil.The vegetative organ biomass increased significantly with the increase of aboveground biomass.There were reproductive thresholds in A.scoparia,and there was a negative correlation between reproductive allocation and aboveground biomass.There was also a negative correlation between the each capitulum mass and aboveground biomass,and between the each capitulum mass and capitulum number of each individual plant,and these correlations reflected the phenotypic plasticity of resource allocation to a certain extent,but did not reach a significant correlation.The height,stem diameter,root length,root weight,and biomass per plant increased with the increase of soil relative humidity,which showed a strong plasticity.The weight of each capitulum mass did not change significantly with the rise of aboveground biomass and the capitulum number of each individual plant,and there was no significant difference between different soil relative humidity and different parts of the single plant.The number and weight of reproductive body in habitats with soil relative humidity>40%were significantly higher than those in habitats with soil relative humidity<30%and 30-40%.With the increase of soil relative humidity,the reproductive allocation increased significantly.The biomass of vegetative and reproductive organs,the weight and quantity of capitulum,the underground biomass and aboveground biomass were all shown significantly positive correlated,so there was a positive trade-off.Therefore,the plasticity of A.scoparia under different soil relative humidity habitats was one of the critical mechanisms for survival and reproduction.The capitulum number and mass of each individual plant of A.scoparia were significantly affected by the content of water-soluble carbon and soil moisture.Reproductive allocation and each capitulum mass were both negatively correlated with water-soluble carbon content and soil moisture content.It could be concluded that water-soluble carbon content and soil moisture content were the main soil factors that affecting the reproductive characteristics of A.scoparia in desert steppe.To sum up,A.scoparia has a high frequency,importance and niche in vegetation structure.It is a key plant in desert steppe.The main reason for the long-term existence of A.scoparia in the vegetation succession of desert steppe is that A.scoparia has many potential allelochemicals,which can inhibit some key species(such as S.capillata)in desert steppe.The life table and survival curve of A.scoparia showed that they had the characteristics of quantity and density.A.scoparia has strong phenotypic plasticity to the soil types in desert steppe,and has a significant response to the increase and decrease of precipitation.Therefore,the survival,allelopathy and the adaptation strategies to precipitation and soil of A.scoparia make it a dominant species and even a key successional species in vegetation pattern of desert steppe. |
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